Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms. Cathode materials that will reversibly intercalate lithium have been studied extensively in recent years for use as electrode materials in advanced high energy density batteries and form the cornerstone of the emerging lithium-ion battery industry. Lithium-ion batteries have the greatest gravimetric (Wh/kg) and volumetric (Wh/L) energy densities of presently available conventional rechargeable systems (i.e., NiCd, NiMH, or lead acid batteries) and represent a preferred rechargeable power source for many consumer electronics applications. Additionally, lithium ion batteries operate around 3.6 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. The excellent reversibility of lithium insertion makes such compounds function extremely well in rechargeable battery applications wherein over one thousand battery cycles can be obtained. In a lithium-ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with the associated electrons traveling in the circuit external to the battery. Layered rock-salt compounds such as LiCoO.sub.2 and LiNiO.sub.2 (1,2) are proven cathode materials. Nonetheless, Co and Ni compounds have economic and environmental problems that leave the door open for alternative materials.
LiMn.sub.2 O.sub.4 is a particularly attractive cathode material candidate because manganese is environmentally benign and significantly cheaper than cobalt and/or nickel. LiMn.sub.2 O.sub.4 refers to a stoichiometric lithium manganese oxide with a spinel crystal structure. A spinel LiMn.sub.2 O.sub.4 intercalation cathode is the subject of intense development work (3), although it is not without faults. The specific capacity obtained (120 mAh/g) is 15-30% lower than Li(Co,Ni)O.sub.2 cathodes, and unmodified LiMn.sub.2 O.sub.4 exhibits an unacceptably high capacity fade. Several researchers have stabilized this spinel by doping with metal or alkali cations (4,5). While the dopants successfully ameliorated the capacity decline, the initial reversible capacity is no better than 115 mAh/g, and the running voltage of the cell is no better than the usual 3.5 V.
Extending the concept of Mn replacement in the spinel, Davidson (6) used &gt;20 mole % Cr and Ni, respectively, and produced 3 V LiM.sub.x Mn.sub.2-x O.sub.4 cathodes that immediately intercalated Li (discharged). Gao (7) discovered that LiNi.sub.x Mn.sub.2-x O.sub.4 has a 4.7 V plateau corresponding to the oxidation of Ni(II), and the capacity of the system 0.ltoreq.x.ltoreq.0.5 is nearly constant over the range 3.5 to 5.0 V.
The higher potential transition was assigned to Ni.sup.+2 .fwdarw.Ni.sup.+4 oxidation, with a corresponding Li.sup.+ deintercalation. Further, the 4.7 V plateau increased with Ni concentration at the expense of the Mn.sup.+3 .fwdarw.Mn.sup.+4 4 V plateau, as predicted, and for LiNi.sub.0.5 Mn.sub.1.5 O.sub.4, which contains only Mn.sup.+4, there was no electrochemical activity at 4 V.
Thackeray and Gummow (4) have developed 4 V doped spinel LiMn.sub.2 O.sub.4 cathode materials of the following composition: Li.sub.1 D.sub.x/b Mn.sub.2-x O.sub.4+.delta., where the dopant, D, is a mono- or multi-valent metal cation. Both Mg.sup.2+ and Co.sup.3+ were claimed as dopants.
More recently, Zhong and Bonakdarpour (5) have developed high voltage (.about.5 V) doped spinel LiMn.sub.2 O.sub.4 cathode materials of the following composition: Li.sub.x+y M.sub.x Mn.sub.2-y-z O.sub.4, where the dopant, M, is a transition metal. Specific examples of M are nickel and chromium.
However, the existing doped spinel cathode materials manifest unremarkable electrochemical stability leading to poor cycle life. Therefore, a need exists to improve the stability of ultrahigh (.gtoreq.5 V) voltage cathode materials and such materials are the subject of this invention. Lithium ion batteries incorporating such materials will outperform existing cell chemistries with respect to running voltage and power densities in particular.